Mumbai University > First Year Engineering > Sem1 > Applied Chemistry 1

Marks: 5M

Year: May 13, Dec13, Dec12, Dec 14, May 15

Compression moulding

It is used for moulding both thermosetting as well as thermoplastic plastics. The raw materials of correct composition are placed between the top and bottom part of the mould which is closed under low pressure. Then high pressure and temperature is applied which is of the range 100-500 kg/cm2 and 100 -200oC respectively to complete the curing process. The cavities are filled with
the fluidized plastics. Finally curing is done either by heating and cooling. After curing, the article is taken out by opening the mould.

Injection moulding

This is used for high speed moulding of thermoplastics. The moulding composition is heated in a suitable chamber connected by a duct leading to the mould. The hot softened molten plastic is injected at a controlled rate into the tightly locked mould by means of a screw or a piston plunger. The mould is kept cold to allow the hot plastic to cure and became rigid. When the material is cured sufficiently, half of the mould is opened to allow the ejection of the finished article.

Drawback of this method is a large number of cavities cannot be filled simultaneously. The temperature used is 90o-260o and pressure used is 100-500 kg/$cm^2$.

Transfer moulding

It is the modification of injection moulding and is suitable for thermosetting plastics. The moulding composition is first plasticized by applying minimum heat and pressure in a chamber outside of the mould. This plasticized material is then injected through an orifice into the mould by a plunger working at a high pressure. It is heated upto curing temperature required for setting. The moulded article is ejected mechanically.

Advantages-

1) Mould cost is low.

2) Finishing of article is not required.

3) Suitable for intricate shapes.

Extrusion moulding

This method is used only for thermoplastics for manufacture of electric wires, cables, tubes, rods etc. having uniform cross-sections. Like injection moulding, the raw material or the moulding composition is forced by a screw conveyor into a heated chamber where it melts and then is forced through a Die having the required shape. The finished product is cooled by atmosphere or by blowing air or by spraying water as it is carried away by a long conveyor.